The objective of the proposal is to characterize renin in aortic tissue and plasma of the spontaneously hypertensive rat (SHR) and to determine if interrelationships exist between the activity of these enzymes and induced regression and spontaneous progression of hypertension. We have demonstrated that as blood pressure increases in the SHR, a fall in aortic renin occurs. This decline in activity was not related to age (3-13 months) since those SHR failing to develop hypertension did not show the decline. Despite this, induced reductions in blood pressure, at least in the developing stages of hypertension in the SHR (4-7 weeks) by pharmacological intervention, also reduced aortic renin activity. The fall in blood pressure was associated with both increases and decreases in plasma renin. In contrast to the SHR, preliminary data indicates a direct relationship between aortic renin and blood pressure in Goldblatt hypertensive rats. Aortic renins (n equals 2), and plasma renins (n equals 6) have been separated on the basis of their isoelectric point. One enzyme (M.W. 42,000) may be common to both aortic tissue and plasma. Despite this, no fall in aortic renin activity was observed in 24 hour nephrectomized rats. Although the existence of inactive renin in aortic and kidney tissue has not been confirmed, three forms of plasma renin, one with a M.W. 150,000 and the other two 40-44,000, can be activated. The above studies are to be continued during the coming year in the SHR during the established phase of hypertension and the characterization of aortic and plasma renins in response to pharmacologic intervention continued.